Method and electrolyte for anodic oxidation coating of aluminum

ABSTRACT

IN THE ANODIC OXIDATION COATING OF ALUMINUM AND ITS ALLOYS AN ELECTROLYTE IS EMPLOYED COMPRISING AN AQUEOUS SOLUTION CONTAINING, BY WEIGHT OF SAID SOLUTION, 1% TO 30% ACETAMIDE, 0.1% TO 10% OXALIC ACID AND 0.01% TO 0.5% SULPHURIC ACID OR ITS MOLAL EQUIVALENT OF A WATER SOLUBLE SULPHATE COMPOUND. DEEPER COLORS ARE ACHEIVED AT LOWER VOLTAGES AND POWER CONSUMPTIONS.

United States Patent Int. Cl. (5231; 9/02 US. Cl. 204-58 6 Claims ABSTRACT OF THE DISCLOSURE In the anodic oxidation coating of aluminum and its alloys an electrolyte is employed comprising an aqueous solution containing, by weight of said solution, 1% to 30% acetamide, 0.1% to 10% oxalic acid and 0.01% to 0.5% sulphuric acid or its molal equivalent of a Water soluble sulphate compound. Deeper colors are achieved at lower voltages and power consumptions.

BACKGROUND OF THE INVENTION The present invention relates generally to improvements in metal treating and it relates particularly to an improved method for the production of integrally colored anodic oxidation coatings on articles of aluminum and alloys thereof and an improved electrolyte composition therefor.

The conventional process for forming a colored anodic oxidized coating on articles of aluminum or aluminum alloys includes the step of immersing the aluminum article, which defines an anode, and a cathode defining electrode into an electrolyte, generally sulfuric acid, and applying a voltage between the article and cathode to effect the formation of an oxide coating on the article. However, the resulting coating is greatly deficient because of its high porosity a consequence of which is a low corrosion resistance. Moreover, an additional separate dye treating step is required. The conventional aluminum anodizing procedure accordingly leaves much to be desired. The applicant has found that many of the drawbacks of the conventional aluminum anodizing methods are overcome by employing as an electrolyte an aqueous solution of acetamide and oxalic acid. While this process produces a hard, non-porous, highly corrosion resistant, colored coating on the aluminum article and obviates the need of any additional dye treatment it possesses certain disadvantages, it requires the use of relatively high voltage and is accordingly accompanied by a relatively high power consumption and the resulting color, although variable depending on the composition of the aluminum alloy, is of relatively pale or light hue.

SUMMARY OF THE INVENTION It is a principal object of the present invention to pro vide an improved method and composition for the treatment of metals.

Another object of the present invention is to provide an improved method and composition for the production of anodic oxidation coatings on surfaces of aluminum and alloys thereof.

Still another object of the present invention is to provide an improved method and composition for producing on aluminum containing articles a hard highly corrosion resistant, colored, anodic oxidation coating.

A further object of the present invention is to provide an improved single step method of the above nature characterized by a relatively low voltage requirement and power consumption, great versatility and adaptability, a wide range of colors and depths of color, and a superior resulting coating.

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In a sense the present invention is based on the discovery that the use, in the formation of an anodic oxidation coating on an aluminum containing surface, of an electrolyte comprising an aqueous solution containing, in addition to acetamide and oxalic acid, a small amount of sulfuric acid or a water soluble sulfate compound which provides sulfate ions in the aqueous solution, produces a hard, nonporous, highly corrosion resistant, deeply colored coating on the aluminum surface, and the voltage requirements between the aluminum surface and the cathode are greatly reduced with a corresponding reduction in power consumption. The present invention accordingly contemplates an electrolyte for the anodic oxidation of aluminum and alloys thereof comprising an aqueous solution of acetamide, oxalic acid, and sulfuric acid or a water soluble sulfate or a combination thereof, as well as the method of treating an aluminum containing work piece comprising immersing the work piece and another electrode in the improved electrolyte and applying a voltage between said work piece and said electrode of a value sufficient to form an oxidation coating on said work piece.

The concentration of the sulfate ion in the aqueous electrolyte solution should be sufiicient to reduce the anodic oxidation coating voltage, advantageously to an initial voltage of less than 70 volts and a final voltage of less than volts, and advantageously constitutes by weight of the electrolyte of 0.01% and 0.5% sulfuric acid or its molar equivalent of a water soluble sulphate compound or a combination thereof with sulfuric acid, that is between about 0.01% and 0.5 of the sulfate ion. Preferably the electrolyte contains between 0.1 and 2.0 grams sulfuric acid per liter of electrolyte or the molar equivalent thereof of the sulfate compound or of said compound in combination with sulfuric acid. The electrolyte advantageously contains, by weight, 1% to 50% acetamide, preferably 30 to 200 grams acetamide per liter of electrolyte, and 0.1% to 10% oxalic acid, preferably 3 to 30 grams oxalic acid per liter of electrolyte. The temperature of the electrolyte in the anodizing procedure, the current density and the anodizing time may be greatly varied depending on the desired coating, and the optimum parameters are easily and readily determined by one skilled in the art. Thus an electrolyte temperature between 15 C. and 25 C. and a current density of the order of 2 a./dm. has been found to be highly satisfactory. The color of the anodic oxidation coating is dependent on the composition of the aluminum alloy of the article being treated.

The present invention is characterized by an increased efficiency consequent to a reduction in the required anodizing voltage and in the power consumption and results in a significant reduction in equipment and costs. The improved process is capable of producing a very wide range and depth of colors without the need of separate dyeing steps. The color of the resulting coating is highly resistant to light and weathering and exhibits very little discoloration even when exposed for long periods to out-of-door infiuences and environments. Thus, articles coated by the present process posses such high wear and corrosion resistance properties that they are highly suitable for outdoor uses and applications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electrolyte of the following composition was pre- 1 1 liter in total amount.

The solution prepared in accordance with the above whereby to optimumly obtain the desired color and coatcomposition was put into an electrolytic cell. Into this ing. solution were immersed a work piece defining aluminum The color of the anodized coating depends on the plate to be treated which served as an anode and another composition of the work piece. The thicker the oxidized aluminum plate which served as a cathode. Three differ- 5 coating, the deeper the color becomes, so that by supplyent types of aluminum were employed as the work piece ing the current selectively for an appropriate period of and subjected in the electrolyte to anodic oxidation under time, desired coloration can be achieved through a single the conditions and parameters set forth in the following step of anodic oxidation. In addition, the coating obtained table, the voltage between the anode and cathode being with the use of the improved electrolyte is tough and regulated in known manner to maintaina constant current 1 high in wear resistance and is substantially free of any density: pores. (Pinhole test results in no deposition of copper Current Voltage range Temperature Time of density of solution anodizin N0. Material (aJdmfl) Starting Final 0 (min.

1 1080 (99.8%, Al) 2 65 84 16-18 45 2 5052 (52 S 2 62 95 17. -18 45 3 6063 (63 S) 2 63 88 18-20 45 Set forth in the following table are the properties of other than by corner effect.) It also has excellent resisthe anodic oxidation coatings formed on the respective 20 tance to light and weather-proof property and does not samples, treated as above: develop cracks even when it is thick. The oxidized coat- Fihn Abrasion thickness Hardness resistance No. Material (11) (Hv) (sec.) Color 1 1080 (99.8% Al) 18. 5 552 2, 933 Slightly yellowish deep gray.

2 5052 (52 S) 16 533 5,773 Dark olive.

3 6053 (63 S) 23. 8 536 5,760 Dark brown.

For comparison purposes an electrolyte similar to that ing in accordance with the present invention can 'be formed described above but without addition of sulfuric acid and in a wider range of colors which covers deeper colors as having the following composition was prepared: compared with the coating produced by anodic oxidation Grams/liter which utilizes an aqueous solution consisting of acetamide Acetamide 80 and oxalic acid. The higher the ratio of the amount of oxalic acid 10 acetamide to that of oxalic acid, the darker color of the Water 1 B a1 ance oxidized article is attained.

""""""""" The improved electrolyte set forth above may be 11 liter in total amounitmodified as earlier described by varying the concentra- Employing the above electrolyte Without sulfuric acid, tions of the acetamide, oxalic acid, and sulfuric acid or three aluminum plate work pieces were treated in the 40 sulfate ion in the manner earlier described. The sulfuric manner described above under the following conditions acid may be substituted in whole or in part by a water and parameters: soluble sulfate salt or compound which provides sulfate The properties of the anodic oxidation coatings of the ions in aqueous solution whereby the sulfuric acid conlast treated samples are as follows: centration is as specified, preferably in the range of 0.1

Film Abrasion thickness Hardness resistance No. Material t) (Hv) (sec.) Color As can be seen from the above examples, by employto 2.0 grams per liter of electrolyte solution. An example ing the improved electrolyte composition in the anodic of a suitable sulfate compound is nickle sulfate, or the like. oxidation of aluminum surfaces a wide range of colors is 0 Whil th h b d ib d f r d b d available in p hues- AS compared to an electrolyte ments of the present invention, it is apparent that nui differs from the lfi electrplyte y In the merous alterations, omissions and additions may be made omission of the sulfate ion, its use provides much darker Without departing f the Spirit th fl shades under identical current and time treating param- Whatis claimed is: eters. the Vintage required. for i Same current 1. An electrolyte for the anodic oxidation of alumdensltles i c9nslderablii.less.wlth the Improved .electro inum and alloys thereof comprising an aqueous solution lyte resulting in a reduction in power consumption and containi b of S 1 t 01 t t 507 f equipment and installation costs. It should be noted, that y 31 e y F 0 0 0 o as the coating on the work piece builds up, the resistance acetamlde to oxahc i and 001% to between the electrodes increases thereby necessitating an 05% of sulphunc acld molal eqmvfalent of Wafer increase in the interelectrode Voltage to maintain a cork soluble sulphate or combination th6f0f W1th'Sl1lfllI1C acid. Stant current density. This Control may be manually 2. The electrolyte of claim 1 wherein said electrolyte performed but is preferably accomplished automatically Contains 30 to 200 grams P liter of Said acetamide, 3 by known electrical regulator networks so that the current to 30 grams per er of Said OXalic acid, and f0 2 density does not vary during the full treatment period grams per liter of said sulfuric acid or the molal equivalent thereof of a water soluble sulfate or a combiuation thereof with sulfuric acid.

3. The method of treating an aluminum containing work piece comprising immersing said work piece as the anode and immersing a cathode in the electrolyte of claim 1 and applying a voltage between said work piece and said cathode of a value sufficient to form an oxidation coating on said work piece.

4. The method of claim 3 wherein said voltage is increased during said treatment.

5. The method of claim 3 wherein the initial voltage applied between said work piece and electrode is below 70 volts.

6. The method of claim 3 wherein the temperature of said electrolyte is between 15 C. and 25 C.

References Cited UNITED STATES PATENTS 3,472,744 10/1969 Kape 204-58 OTHER REFERENCES The Anodizing of Aluminum in Electrolytes Based on 10 Organic Solvents, Plating, January 1968, pp. 26-34.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

